1. Field of the Invention
The present invention relates to a disk drive having a tracking position detection device enabling position detection of fine pitches by using one or two pairs of position detection signals of mutually inverted phases, a position detection device for position detection of a head of a hard disk drive and other applications requiring high precision, and a method of correction of the position detection signals.
2. Description of the Related Art
As a preferred circuit for position detection of, for example, a head of a disk drive, Japanese Unexamined Patent Publication (Kokai) No. 2-226012 discloses an optical position detection circuit configured as shown in FIG. 7.
The position detection circuit 100 is connected to four light receiving elements (photo diodes) 201 to 204 as shown in FIG. 7. The photo diodes 201 to 204 are struck by light from a light emitting diode 205 controlled by a drive circuit 206. Between the light emitting diode 205 and the photo diodes 201 to 204, while not particularly shown, are interposed two light blocking plates each provided with a plurality of rectangular slits. One is called a xe2x80x9cscalexe2x80x9d and has slits repeatedly arranged at a certain pitch in its longitudinal direction. The other is called a xe2x80x9creticlexe2x80x9d, is a light blocking plate which moves in one direction together with a head, and has four slits arranged in a positional relationship a little different from the slits of the scale. The pitches and positional relationship of the slits on the scale and reticle are set so that the superimposed widths of the slits as seen from the light receiving side repeatedly increase and decrease substantially linearly when the head is moved at a constant speed and so that the phase of the repetition is shifted 90 degrees each. Thus, the four photo diodes 201 to 204 output triangular wave-shaped position detection signals having phases shifted by 90 degrees as the head moves.
The position detection circuit 100 shown in FIG. 7 comprises first to fourth detection circuits 101 to 104, a selector 110, a maximum value detection circuit 120, a minimum value detection circuit 130, analog-to-digital (A/D) converters 121 and 131, a microcomputer 140, an erasable programmable read only memory (EPROM) 150, a digital-to-analog (D/A) converter 160, an input-output (I/O) port 170, etc.
The first to fourth detection circuits 101 to 104 are amplifying circuits of a current/voltage conversion type having a function of adjusting a gain and a voltage offset.
The detection circuits 101 to 104 each comprise, as representatively illustrated by the first detection circuit 101, two operational amplifiers 105 and 106, 13 resistors R1 to R13, and eight switches S1 to S8.
Non-inverted inputs of the operational amplifiers 105 and 106 are connected to an anode of a photo diode 201, while the connecting point is connected to a supply line of a constant voltage +B. An inverted input of the first operational amplifier 105 is connected to a cathode of the photo diode 201. Between the inverted input and an output of the operational amplifier 105 are connected in series five resistors R1 to R5. The switches S1 to S4 are respectively connected in parallel to the two ends of the four resistors R2 to R4 among them.
An output of the operational amplifier 105 is connected to an inverted input of the operational amplifier 106 via the resistor R6. A feedback resistor R7 is connected between the inverted input and an output of the operational amplifier 106. Further, five resistors R8 to R12 are connected in series between the inverted input of the operational amplifier 106 and a ground potential. The switches S5 to S8 are respectively connected in parallel to the two ends of the four resistors R9 to R12 among them. A mid-point of connection between the resistor R8 closest to the inverted input of the operational amplifier 106 and the second closest resistor R9 is connected to a supply line of a constant voltage +B via the resistor R13.
The output of the operational amplifier 106 is connected to an output terminal To of a position detection signal. Also, a switch circuit including the switches S1 to S4 is connected to a first control terminal Tc1, while a switch circuit including the switches S5 to S8 is connected to a second control terminal Tc2.
In the first to fourth detection circuits 101 to 104 configured in this way, the switches S1 to S4 are appropriately switched in accordance with a control signal transmitted from the microcomputer 140 via the I/O port 170 and input from the first control terminal Tc1, whereby a feedback resistance value of the operational amplifier 105 changes and the gain is adjusted. Therefore, it is possible to adjust the amplitude of the position detection signal output from the output terminal To. Hereinafter, the adjustment of the amplitude of the position detection signal will be referred to as xe2x80x9cgain adjustmentxe2x80x9d.
On the other hand, the switches S5 to S8 are appropriately switched in accordance with a control signal transmitted from the microcomputer 140 via the I/O port 170 and input from a second control terminal Tc2, whereby the potential of the inverted input of the operational amplifier 106 changes. Therefore, the direct current voltage level of a position detection signal output from the output terminal To can be adjusted. Hereinafter, the adjustment of the direct current voltage level of the position detection signal will be referred to as xe2x80x9coffset adjustmentxe2x80x9d.
At the time of the gain adjustment and the offset adjustment, one of the four position detection signals is selected by a selector 110 controlled by the microcomputer 140. The selected position detection signal is input to the maximum value detection circuit 120 and the minimum value detection circuit 130, which detect the maximum value and minimum value of the amplitude. The detected values are converted to digital signals by the A/D converters 121 and 131 and then input to the microcomputer 140.
Such detection of a maximum value and a minimum value of the amplitude is performed for all position detection signals by successively switching the selector 110.
The microcomputer 140 calculates a difference and sum of the maximum value and the minimum value of the amplitude for each of the position detection signals. Based on the calculation results, the microcomputer 140 controls the switches S1 to S4 in the detection circuits and performs gain adjustment so that the differences between the maximum value and the minimum value align at a constant value. Also, the microcomputer 140 controls the switches S5 to S8 in the detection circuits and performs offset adjustment so that the sums of the maximum value and the minimum value align at a constant value. The EPROM 150 stores control data of the switches at the time of the gain adjustment and the offset adjustment.
The microcomputer 140 then converts the control signal of a voice coil motor 210 from a digital to analog format, then transmits the same to the motor drive circuit 211.
The position detection device 100 configured in this way can use the four position detection signals having phases successively shifted by 90 degrees to perform position detection by a pitch four times finer than the pitch of the slits provided on the scale.
Also, under control by the microcomputer, gain adjustment and offset adjustment of the position detection signals can be automatically performed based on control data stored in advance.
Summarizing the problem to be solved by the present invention, the position detection device 100 of the related art had the resistors R1 to R5 and switches S1 to S4 for the gain adjustment and the resistors R8 to R12 and switches S5 to S8 for the offset adjustment in the first to fourth detection circuits 101 to 104 provided corresponding to the four position detection signals so had a large number of components and was large in the size of the detection circuits is large. If desiring to further improve the precision of the gain and offset adjustment, it is necessary to provide still larger number of resistors and switches.
Also, the maximum value detection circuit 120 and the minimum value detection circuit 130 are also necessary.
Therefore, it is difficult to make the position detection device 100 of the related art smaller in size.
Furthermore, normally, there is some difference in characteristics among resistors and among switches. Further, the characteristic values vary in accordance with differences of temperature characteristics. Therefore, there is a disadvantage that the precision at the time of gain and offset adjustment easily declines.
In the position detection device of the related art, it is not known at first how much the gain and offset are deviated, so the amounts of deviation of the gain and offset are calculated from the maximum value and minimum value, then the position detection signal is corrected by controlling the detection circuits based on the calculation results. After that, a motor is driven based on the corrected position detection signal. Accordingly, correction of the position detection signal takes time. This is disadvantageous in terms of the production efficiency and production costs. Also, when the position detection signal is corrected during use, the time taken by the correction becomes a cause reducing the response in control.
An object of the present invention is to provide a position detection device comprising a small number of components and therefore advantageous in terms of reduction of size and cost, not requiring a long time for correction, and capable of correcting the position detection signal simply and with a high precision and a method of correction of a position detection signal.
According to a first aspect of the present invention, there is provided a disk drive comprising: a head arm having a head; a motor for rotating the head arm on an axis thereof to move the head in a direction across a track on a disk; a tracking position detection device for detecting a tracking position of the head by detecting a position of the head arm portion moving with the head; and a motor drive portion for driving the motor in accordance with the tracking portion detected by the tracking position detection device, wherein the tracking position detection device comprises, a detector for detecting a position of the head arm portion and outputting two position detection signals having a predetermined phase difference as the head moves; and a signal processor for performing signal processing based on the two position detection signals, detecting the tracking position and outputting the detection result to said motor drive portion.
Preferably, the detector outputs two position detection signals having a phase difference of 180 degrees along with movement of the head and the signal processor obtains an offset voltage level by averaging the two position detection signals and shifts a position detection signal so that a reference voltage level of a center of amplitude matches the obtained offset voltage level.
Preferably, the detector outputs four position detection signals having phases successively shifted by 90 degrees along with movement of the head and a signal processor obtains an offset voltage level by averaging the two position detection signals and shifts a position detection signal so that a reference voltage level of a center of amplitude matches the obtained offset voltage level.
According to a second aspect of the present invention, there is provided a position detection device, comprising a detector for detecting a position an object to be detected and outputting two position detection signals having a phase difference of 180 degrees along with movement of the object and a signal processor for obtaining an offset voltage level by averaging the two position detection signals and shifting the position detection signals so that reference voltage levels of centers of amplitude match with the obtained offset voltage level.
According to a third aspect of the present invention, there is provided a position detection device, comprising a detector for detecting a position of an object to be detected and outputting four position detection signals having phases successively shifted by 90 degrees along with movement of the object and a signal processor for obtaining an offset voltage level by averaging the four position detection signals and shifting the position detection signals so that reference voltage levels of the centers of amplitude match with the obtained offset voltage level.
Preferably, the signal processor corrects amplitudes of the position detection signals by using two position detection signals having a phase difference of 90 degrees among the four position detection signals.
For example, the signal processor obtains the maximum value and minimum value of the amplitude from the two position detection signals shifted in phase by 90 degrees and increases or reduces the amplitude of a position detection signal so as to give the obtained maximum value and the minimum value. In that case, preferably, the signal processor corrects the offset, obtains the maximum value, and obtains the minimum value by subtracting from the shifted reference voltage the voltage difference between the obtained maximum value and the shifted reference voltage.
Preferably, the signal processor obtains the maximum value and minimum value of amplitude from the two position detection signals having a phase difference of 90 degrees and expands or reduces the amplitude of a position detection signal to give the obtained maximum value and minimum value.
In that case, preferably the signal processor averages the added result within a predetermined phase range and regards the obtained average value as the maximum value.
Alternatively, the signal processor obtains a first added value by adding absolute values of a first position detection signal and a second position detection signal having a phase 90 degrees advanced with respect to the first position detection signal, obtains a second added value by adding absolute values of a third detection signal having a phase 90 degrees delayed with respect to the first position detection signal and the first position detection signal, and averages the first and second added values to obtain the maximum value.
Alternatively, the signal processor obtains a first added value by adding absolute values of a first and second position detection signals having a phase difference of 90 degrees, obtains a second added value by adding absolute values of a third and a fourth position detection signals respectively having a phase difference of 180 degrees with respect to the first and second position detection signals, and averages the first and second added values to obtain the maximum value.
Alternatively, the signal processor corrects the obtained maximum value. For example, the signal processor obtains points where two position detection signals having a phase difference of 90 degree cross center lines of amplitude and corrects the maximum value based on the crossing points.
Alternatively, the signal processor obtains the maximum value, then obtains points where two position detection signals having a phase difference of 180 degrees cross center lines of amplitude, and confirms whether phase differences between the crossing points and the maximum value are within a predetermined range.
Preferably, the detector comprises a light emitter; a reticle having a plurality of slits; a scale having a plurality of slits repeating at a constant pitch and relatively moving with respect to the reticle along with movement of an object being detected; and a light receiver for receiving light from the light emitter passing through the reticle and the scale and outputting four position detection signals having phases successively shifted by 90 degrees forming triangular waves due to a change of relative positions of the slits along with movement of the scale.
Preferably, between the detector and the signal processor is provided a detection circuit for amplifying the four position detection signals from the detector by a constant amplifying rate and a constant direct current voltage level.
Preferably, the position detection device further comprises a selector for selecting one of the four position detection signals and an A/D converter for converting a position detection signal selected by the selector from an analog signal to a digital signal and outputting the result to the signal processor.
Further, the position detection device further comprises a memory for storing at least one of the position detection signals and calculation results of the signal processor.
The disk drive or position detection device configured in this way converts the position detection signal before correction to a digital signal by the A/D converter and inputs it to the signal processor.
The signal processor first corrects the offset. Specifically, it averages all of the one or two pairs of position detection signals of inverted phases. As a result, the alternating current voltage components are canceled out and an average value of a direct current voltage level (an offset average value) is finally obtained. Since the offset average value can be regarded as an approximate offset voltage value for the position detection signals, the position detection signals are shifted so that the reference voltage level of the center amplitude matches with the calculated offset average value.
Next, the signal processor obtains the maximum value for amplitude correction. Specifically, there are cases of using two position detection signals having a phase difference of 90 degrees, using three position detection signals consisting of any one position detection signal and two position detection signals with phase differences of xc2x190 degrees with respect to that signal, and using four position detection signals having phases successively shifted by 90 degrees. In all of these cases, basically the absolute values between two signals having a phase difference of 90 degrees are added. As a result of the addition, an almost constant voltage alternately becoming the maximum values of the two signals every time the phase advances by 90 degrees can be obtained. There is no problem if the maximum values of the two signals are the same, but when they are different, they are further averaged.
Then, the signal processor subtracts the difference between the maximum value and the reference voltage level from the reference voltage level to obtain a minimum value.
The difference between the maximum value and the minimum value obtained as explained above can be regarded as an approximate amplitude of the position detection signals.
Since it is important for position detection signals that an offset value and an amplitude match between signals, it is preferable to then drive for example a motor etc. based on the position detection signal corrected by the approximate offset value and amplitude.
Note that when strictly correcting signals, it is preferable to correct the obtained maximum value and further adjust the offset. Specifically, for example, it is preferable to obtain crossing points of signals having a phase difference of 90 degrees with a center line of amplitude and to obtain a maximum value again by using the crossing points. Also, it is possible to examine how much the crossing points of two signals having a phase difference of 180 degrees match with a phase of a maximum value and a minimum value of a signal having a phase difference of 90 degrees with respect to the two signals and confirm the condition of the offset adjustment by this.
In the present invention, as explained above, the gains and direct current voltage levels in the detection circuits are made constant and offset adjustment and amplitude adjustment on output position detection signals are all performed in a signal processor.